The present invention relates to a process for catalytic treatment of a polysilazane containing, on average, at least two hydrocarbon groups having aliphatic unsaturation in each molecule.
Organopolysilazanes, hereinafter referred to as polysilazanes, are well known products which take the form of monomers, oligomers and polymers which may be cyclic or linear, and may be in the form of resinous polymers. The polysilazanes can be prepared according to a wide variety of processes, using an extensive range of starting materials.
These polysilazanes can, in particular, be shaped and can be pyrolysed in the form of Si.sub.3 N.sub.4, SiC or mixtures thereof. Another method of manufacturing silicon nitride consists of reacting chlorosilanes (SiCL.sub.4, HSiCl.sub.3 and H.sub.2 SiCl.sub.2) with ammonia at high temperature and in the vapour phase. This method leads directly to a powdered product, making it difficult to manufacture articles in the form, in particular, of fibers. Polysilazanes, in contrast, can be spun in continuous fibers, the pyrolysis of which leads to ceramic fibers.
Polysilazanes can be prepared in the form of thinner or thicker films or of moulded solid articles, and used as a binder for cermaic fiber or carbon fiber, and as a sintering binder for porous ceramic articles.
It is, nevertheless, difficult to shape these polysilazanes easily and economically in the form of fibers or coatings which, after pyrolysis, give ceramic products in the form of fibers, films, thinner or thicker coatings and molded articles.
In U.S. Pat. No. 3,853,567, an attempt was made to provide a solution to the above problem by manufacturing shaped articles such as fibers containing silicon carbide, silicon nitride or mixtures thereof with other ceramic products, by performing a first heat treatment of a polysilazane at a temperature between 200.degree. C. and 800.degree. C., to obtain a fusible carbosilazane resin which can be spun in the molten state and then pyrolyzed at a temperature of between 800.degree. C. and 2000.degree. C.
The above patent admittedly represents a substantial advance, but it has the dual disadvantage of requiring a first heat treatment at a temperature which can already be very high (200.degree.-800.degree. C.) and of working a carbosilazane in the molten state under anhydrous conditions and in an inert atmosphere. In addition, the yield by weight of ceramic can be insufficient.
In Japanese Patent Application No. KOKAI 77/160,446, a process is described for polymerization of high molecular weight organopolysilazanes using acid earths as a catalyst for treatment of the organopolysilazanes. This process, however, has the major disadvantage of requiring separation of the solid catalyst by filtration, involving the use of solvents in the case of polymers which attain high viscosities.
Moreover, the above-cited Japanese patent application exludes the possibility of using polysilazanes possessing.dbd.SiH groups, although, without giving examples, it does not exclude the possibility of using polysilazanes possessing unsaturated aliphatic hydrocarbon groups linked to the silicon atoms, such as alkenyl groups, for example vinyl or allyl groups. Nevertheless, the teaching of this patent application is limited to a polymerization reaction by opening silazane bonds.
U.S. Pat. No. 3,007,886 describes a process for treating polysilazanes with metal salts of monocarboxylic acids which are soluble in hydrocarbon solvents. U.S. Pat. No. 3,187,030 describes a process for treating polysilazanes with particular metal salts of strong inorganic acids, the catalysis being provided by the metal cations which perform the role of Lewis acids. The teaching of these two patents is limited to a polymerization reaction by opening silazane bonds.